FR2560182A1 - Ceramic material based on silicon nitride and on a hard refractory main phase - Google Patents

Abstract

The material is characterised in that it comprises an alpha phase of general formula Mx(Si,Al)12(O,N)16 in which M is at least one of the elements consisting of lithium, calcium, yttrium and the other lanthanides and 0.1 < x < 2, a beta phase of general formula: Si6-zAlzOzN8-z, in which 0 < z < 4.2, an intergranular phase containing sintering adjuvants such as the oxides of M, preferably yttrium oxide, and a phase of hard refractory principles containing carbides, nitrides and/or carbononitrides of titanium, niobium, tantalum, vanadium, chromium, molybdenum, tungsten, hafnium and/or zirconium, whose sintered structure contains, by volume, 5 to 60 % of a first component I forming the hard refractory principle and the remainder consisting essentially of a second component II comprising 10 to 70 % of alpha phase, 20 to 90 % of beta phase and 0.1 to 20 % of intergranular phase.

Description

 The present invention relates to a ceramic alloy having superior qualities, particularly for use in cutting tools. In particular, the invention relates to a type of cutter material which is based on sialon (silicon, aluminum, oxygen, nitrogen) in which the wear resistance is improved by the addition of hard refractory principles and the toughness is increased by making this material comprise two different crystalline phases.

 Alumina-based ceramic cutting inserts are characterized by superior chemical stability, which allows their use in cutting operations at high cutting speeds without being oxidatively or chemically dissolvable. - However, a serious limitation in the use of alumina materials lies in the high sensitivity of these materials to mechanical and thermal shock, which severely reduces their potential for use in intermittent cutting operations, for example in milling.

 Silicon nitride materials have been tried in cutting tools in recent years, and have shown very good toughness because of their low coefficient of thermal expansion. Their application possibilities, however, are limited by relatively high wear due to their high reactivity with iron. The wear resistance can be considerably improved by combining silicon nitride with other hard principles which have a higher hardness and chemical stability. Such materials have already been described for example in the publication "Por Met" t1978): 2, pages 48 to 52, in which mention is made of a cutting material based on silicon nitride with addition of titanium carbide.In the same publication, (1981): 7, pages 73 to 77, silicon nitride nitride additions were studied. Similar cutting materials are described in Japanese Patent Application No. 56-32377 of April 1, 1981, wherein the addition consists of 5 to 40 wt% TiC, Ti (C, N) or TiN, and European Patent Applications 35777 and 95129 in which the additions are carbides and / or nitrides of metals which belong to groups IVB, VB and VlB of the Periodic Table of Elements. It is characteristic for these materials that, in addition to refractory carbide or nitride, they consist of a single crystalline phase of silicon nitride or sialon beta and possibly another phase containing a large amount of sintering aids, such as, for example than yttrium oxide, and which can be amorphous.

 According to the invention it has been found, quite surprisingly, that the cutting properties can in some cases be considerably improved for a material essentially based on silicon nitride, a carbide / nitride refractory phase and a a phase containing sintering aids if the silicon nitride component - which is actually the sialon - is present in two crystalline forms: alpha and beta.

The alpha phase is a hexagonal phase of general formula: Mx (Si, Al) 12 (O, N) 16, in which M = Li, Ca,
Y or other lanthanides and x 2.

The beta phase is a hexagonal phase of general formula
Si6-zAlzOzN8-z wherein O <z <4.2.

The presence of the alpha phase increases the hardness measured at room temperature, but it also has a favorable influence on the resistance to plastic deformation (see below) because, probably, the hot hardness is also increased in a favorable direction. The plastic deformation of the cutting edge can be observed under conditions for which the edge is subjected to a high temperature, i.e. at a high cutting speed and / or feed rate. The plastic deformation leads to the fomlaLion of fissures on the cutting aryl of the cutting insert, which causes a deficiency of the tool when these cracks evolve. This type of crack formation has been described for example in the document
Met.Tech 10 (1983): Dcc p 482-9 (Bhattacharyya et al., "Wear mechanisms of sialon ceramics tools when machining nickel base materials"). Surprisingly, it has been found that the presence of sialon alpha greatly improves the ability of the fabric to withstand plastic deformation, thereby allowing higher cutting speeds and feed rates without the risk of catastrophic fractures.

 The improvement of the properties is mainly manifested by a considerable increase in the resistance of the cutting edge to the formation of chips in certain workpiece materials. This advantage has been observed particularly in certain nickel-based high temperature materials because of the high cutting temperatures. However, the same phenomenon also occurs in other workpiece materials if certain values of cutting speeds and feed rates are exceeded.

 The present invention therefore relates to a material which is characterized in that it contains an alpha phase and a beta phase of the sialon type and, in addition, an intergranular phase, generally in the vitreous state, but which can also consist of phases crystal. In addition, there is a phase of refractory hard principle consisting of carbides, nitrides or carbonitrides of metals belonging to groups IVB, VB and / or VIE of the periodic table of elements.

 The coexistence of the alpha and beta phases is possible for certain compositions of matter and has been described for example by HKPark et al in the article "Alpha-sialon ceramics", Science of Ceramics ", Vol 10 pp. 251-256, H Ilausner (Ed) and also in British Patent Application 2 118 927 A. This patent application relates to silicon oxynitride and aluminum ceramic cutting materials having an alpha phase and a beta phase of Si-Al- The material mentioned above, however, contains no carbide, nitride or refractory carbonitride.

 The properties of the material which are specific to the invention are apparent from the following description.

The presence of a refractory phase is intended to improve the wear resistance properties. This phase is present in the material as a finely dispersed discrete phase. In the following description, titanium nitride is used as the refractory phrase because it is a hard principle often used but it is evident that similar improvements in properties can be obtained by other kinds of hard refractory principles, such as Examples of carbides, nitrides or carbonitrides of metals titanium, tantalum, chromium, molybdenum, niobium, zirconium, hafnium and tungsten, or their mixtures. The presence of oxygen in hard refractory principles has not proved unfavorable but on the contrary, they can have a favorable influence on densification during sintering. From the point of view of manufacture, nitrides are preferred because they make it possible to obtain a densely sintered body using sintering without pressure. By using carbides, sintering can often not be done without pressure.

 According to the invention, the necessary presence of the alpha phase and the sialon beta phase is obtained by regulating the starting materials in such a way that a relatively small amount of alumina and silicon dioxide and a larger amount of Aluminum nitride and yttrium oxide (or corresponding materials) leads to an increased fraction of sialon alpha in the structure.

The presence of sialon alpha leads to greater hardness without noticeable drop in transverse rupture strength values.

 Sintering aids, such as preferably yttrium oxide, have been used to make the above product, but similar results can be obtained with one or more oxides, for example strontium, cerium, lanthanum and others. elements of the lanthanide series.

The use of yttrium oxide as a sintering aid results in an intergranular phase which is essentially vitreous but which may also comprise other phases, for example YAG (a cubic phase of formula
Y3AL5012), YN-alpha-wollastonite (a monocyclic phase of formula YSiO2N), YAM (monocyclic phase of formula Y4Al209) and N-YAM (a monocyclic phase of formula
Y4Si207N2)
According to the invention, the product consists of several phases and can be prepared according to the description which follows.

 Silicon nitride, aluminum nitride, alumina and silicon alkoxide, the oxides being impurities in the nitrides, are reacted with an oxide of at least one of the following elements: : yttrium, scandium, cerium, lanthanum or other metals of the lanthanide series For appropriate compositions of materials, two sialon-like phases are obtained, and first a hexagonal phase (beta phase) which obeys the general formula Si6 in which 0 <z <4.2 For high values of z (z> 1.5), a decrease in toughness is obtained, whereas for low values of z the composition of the material can be reduced. make difficult the dense sintering without pressure.

 The lower limit of the value of z is appropriately chosen so that sintering without pressure becomes possible. The exact lower limit value of z, for which sintering without full density pressure is still possible, is difficult to determine since it may depend on the nature of the refractory phases.

Thus, it has been found that it is possible, with material compositions comprising, for example, titanium nitride as hard refractory principle, to sinter without pressure with low values of z, for example 0.1, which was not possible. not possible without such an addition. Therefore, hard refractory principles, in addition to giving the material combinations a high wear resistance, can contribute to effective pore closure during sintering. The exact nature of the course of these phenomena is not known, but, most certainly, the impurities in carbides and refractory nitrides play a certain part in this respect.

 The second phase of sialon type (alpha phase) which is also hexagonal, is represented by the general formula Mx (Si, Al) 12 (O, N) 16, in which M can be for example one or more of the elements constituted by the lithium, calcium, yttrium and other lanthanides, with 0.1 <x <2. The oxides of these metals, in addition to the formation of the alpha phase, also cause the formation of a glassy base. the sintered product.

It has been found that this vitreous phase is necessary for dense sintering to be possible without pressure.

If the amount of glassy phase is high, it can cause deterioration of the properties at high temperature. However, the amount of vitreous phase can be reduced by a rather low addition of yttrium oxide, or a corresponding addition of other compounds, or by a heat treatment in which part of the glassy phase is converted into crystalline form, for example YAM, YAG, N-YAM or YN-alpha wollastonite. The hard refractory phase which consists of carbides, nitrides or carbonitrides of titanium, vanadium, chromium, molybdenum, tungsten, hafnium and / or zirconium is added in the form of a raw material in powder with a small particle size to give a homogeneous dispersion in the matrix above.

The amount of starting material is chosen such that the sintered structure contains 5 to 60% by volume of a phase of hard refractory principles in the form of at least one carbide or nitride of the refractory metals mentioned previously in a matrix comprising 10 to 70% by volume of an alpha phase of the sialon type, 20 to 90% by volume of a beta phase of the sialon type and a vitreous phase constituting 0.1 to 20% by volume and which can be partly crystalline
In general, the material contains at most 40% by volume of alpha phase. In addition, the contents of the intergranular phase in the matrix are normally at most 15% by volume. The sintered structure generally contains at most 45% by volume of a phase of hard principles which, as mentioned above, consists of at least one nitride, usually one of Litane's liquor.

Of course, the structura of the subject matter can in addition be obtained in a different way from that described above. For example, in US Pat. No. 4,127,416, the use of silicon and aluminum oxynitrides of the polytype type, instead of aluminum nitride, has been described for the preparation of the beta type slalon.
Example 1
A mixture comprising 85% Si 3 N 4 (of which about 2% is SiO 2), 1.5% Al 2 O 3, 7% AlN (of which 2 to 3% is Al 2 O 3) and 6.5% Y 2 O 3 - which makes 85% of the total composition - was mixed with 15% TiN, ground in propanol, dried, pressed to form test bodies and sintered in a nitrogen atmosphere at 17750 C. X-ray analysis showed a strong reflection of the alpha and beta sialon and a mean reflection of TiN. The material showed a TiN phase with a very small particle size and a homogeneous dispersion, together with an alpha + beta phase containing a certain portion of vitreous phase (<10%). A microporosity was observed.

Example 2
The same mixture was used as in Example 1, but the sialon composition constituted 70% of the total weight. The rest consisted of 30% TiN. The sintering was carried out at 17900 under a nitrogen atmosphere for one hour. A uniform structure with a small particle size and a low microporosity was obtained.

The density was 3.55 g / cm. X-ray diffraction measurements showed strong reflection of TiN and sialon beta and mean sialon alpha reflection.

The hardness was 1550 HV1.

Example 3
62% of Si 3 N 4 (containing about 2% SiO 2) was mixed with 15% TiN, 16% of a type 21 polyphase (approximate formula SiAl 160 O 6, containing about
5% AlN), 5% Y 2 O 3 and 2% Al 2 O 3. The mixture was ground in a vibratory mill filled with propanol, in which an additional 1.2% (calculated for the dry powder without a pressing agent) was ground into a body. The powder was dried, pressed to form test samples and sintered in the same manner as in Example 1, that is, at 17750 C. A dense and uniform material of a density was obtained. of 3.32 g / cm and a hardness of about 1560 HVl. An insignificant microporosity was obtained. X-ray examination revealed a strong reflection of alpha and beta siajon and a mean reflection of TiN.

Example 4
The materials prepared according to Examples 1 and 2 were tested in a long-running operation on steel SS 2541-03 - (Swedish standard), and sidewall wear (VB in mm) was measured, and crater wear (KT in ym) These measurements were made as a function of working time in minutes.Two test materials were used as reference material for the test: Sandvik CC 680 and Sandvik CC 650 (containing
A1203 and 30% by weight of Ti (C, N). The test was conducted with the cutting insert and according to the operating conditions below.
Plate geometry: ISO SNGN 120416, chamfer 0.2 mm / 200
Cutting speed: 100 m / min
Feed rate: 0.36 mm / rev
Depth of cut: 2 mm.

 In FIGS. 1 and 2, the wearings VB and KT are represented by the mean values of two tests, FIG.

1 showing sidewall wear VB and FIG. 2 showing wear (KT) by cratering as a function of time. In these
Figures, the various curves represent the following topics
Variant 1: CC 680,
Variant 2: material of example 1,
Variant 3: Subject of Example 2
Variant 4: CC 650.

 In addition, the toughness was tested in a specially formed molded test body (SS 0125), the blank having been exposed to high mechanical loads and thermal shocks.

Another cutting test was conducted under the following conditions
Plate geometry: ISO SNGN 120416, chamfer 0,2 irtin x 200
Cutting speed: 300 m / min
Feed rate: 0.5 mmXtr Cutting depth 3 3 mm
In this test, the lifetime of the resistance was determined by observation of the failure of the wafer. The average life span, which can be obtained from the frequency of accumulated fractures in the
Fig. 3, using the same grades or variants 1 to 4 of materials as in Figs 1 and 2, was determined from 15 tests. The following results were obtained:
CC 680 7.93 mn
Example 1 4.87 mn
Example 2 6.00 mn
CC 650 1.20 min
It appears from the machining tests to study the wear resistance and toughness that the material according to the invention is characterized in that it allows with success to combine the superior resistance to wear of the materials. oxide ceramics with the superior toughness of ceramic materials based on silicon nitride. As can be seen in Figs 1 and 2, the wear curves are in perfect agreement with the oxide material, while the toughness (Fig. 3) matches perfectly with the nitride based materials. silicon,
Therefore, the material according to the invention has a field of application considerably wider than the ceramic materials already known.

Claims (5)

 1. ceramic material based on silicon nitride and a phase of hard refractory principles, characterized in that it comprises an alpha phase of general formula Mx (Si, Al) 12 (O, N) 061 in which M is one of the elements consisting of lithium, calcium, yttrium and other lanthanides and 0.14 x 2, a beta phase of general formula Si6 ~ zAlzOzN8 ~ z, in which O <z <4.2, an intergranular phase containing sintering aids such as M oxides, preferably yttrium oxide, and a phase of hard refractory principles comprising carbides, nitrides and / or titanium, niobium, tantalum, vanadium, chromium, molybdenum, tungsten, hafnium, and / or zirconium carbonitrides, the sintered structure of which contains, by volume, 5 to 60% of a first component I constituting the hard refractory ingredient and the the remainder consisting essentially of a second component II having 10 to 70% of alpha phase, 20 to 90% of beta phase and 0.1 to 20% of intergranular phase.  Ceramic material according to claim 1, characterized in that component II contains at most 40% by volume of alpha phase.  Ceramic material according to one of the preceding claims, characterized in that the sintered structure contains at most 45% by volume of hard principle phase.  4. Ceramic material according to one of claims 1 to 3, characterized in that the component II contains at most 15% by volume of intergranular phase.  5. Ceramic material according to one of claims 1 to 4, characterized in that the hard component phase consists of at least one nitride, preferably titanium nitride.